Our recent derivation of embryonic stem (ES) cell lines from rhesus monkeys, common marmosets, and humans has widespread implications for human developmental biology, drug discovery, drug testing, and transplantation medicine. After months or years of growth in culture dishes, these cells retain the ability to form cells ranging from heart muscle to nerve to blood--potentially any cell type that makes up the body. The proliferative and developmental potential of human ES cells promises an essentially unlimited supply of specific cell types for in vitro experimental studies and for transplantation therapies for diseases such as heart disease, Parkinson's disease, and leukemia. In the long run, however, the greatest legacy for human ES cells may be not as a source of tissue for transplantation medicine, but as a basic research tool to understand the human body. This new tool will be particularly important for lineages that differ significantly between humans and mice. The Exploratory Center for Human Embryonic Stem Cell Research proposed here will begin to focus on using human ES cells to study questions in basic human biology that are not easily addressed by other approaches. A center core will provide human ES cell tissue culture support, including media preparation, quality control, fibroblast preparation, and routine SKY chromosomal analysis of cultured human ES cells. In the final year of funding, a small, focused workshop will be organized to bring together human ES cell researchers and other basic biologists to identify challenges and opportunities in the human ES cell field. We propose the following Pilot Projects: [unreadable] [unreadable] (I) We will analyze the transciptional changes in one important early lineage: the transition from human ES cell to primitive ectoderm to neural ectoderm to differentiated neurons. (II) We will analyze the transcriptional control of how BMP4 causes human ES cells to exit self-renewal and differentiate to trophoblast, using expression cloning strategies and iRNA to add and subtract function. (III) We will use a biochemical approach, coupled with cutting-edge mass spectrometry and proteomics, to identify those factors produced by fibroblasts that mediate human ES cell self-renewal. [unreadable] [unreadable]